Hi my friends, I will close this „Yamato did not join the crew“ bullshit.
My explanation:
First of all, Yamato wanted always to join Ace in the first place but she couldn’t because of the explosive Handcuffs.
Than she accepted this and waited for Luffy that he comes to Wano Kuni because of Odens prophecy.
The second point is that her joining would fit exactly Odens joining to Roger‘s Crew. Oden joins the future Kaizoku. And now the new „Self-declared Oden“ joins the future Kaizoku.
Luffy would never reject someone which had a friendship with Ace and whom Ace promised to come back.